Tests for water

Water can be detected using a substance that changes colour when it takes up water. A substance with no water in its structure is called anhydrous. When it takes up water, it forms a hydrated substance, which may have a different colour.

Two anhydrous compounds can be used to test a sample for water:

• Anhydrous cobalt(II) chloride is blue. It turns pink when water is present.
• Anhydrous copper(II) sulfate is white. It turns blue when water is present.

In each test, the colour change occurs because the anhydrous compound takes up water and forms a hydrated substance. A positive result shows that the sample contains water; it does not show that the sample is pure.

Purity of water

A pure substance has a fixed melting point and a fixed boiling point at a given pressure. Pure water melts at $$0,^{\circ}\mathrm{C}$$ and boils at $$100,^{\circ}\mathrm{C}$$ at standard atmospheric pressure.

Dissolved substances or other impurities can change these temperatures, so a water sample can be tested by measuring its melting point and boiling point. If the measurements differ from those of pure water under the same pressure, this suggests that the sample is impure. Measurements close to the expected values support purity, but may not reveal very small amounts of impurity.

Distilled water in practical chemistry

Distilled water contains far fewer dissolved salts and other chemical impurities than tap water. It is therefore used in practical chemistry because it is less likely to introduce unwanted substances, interfere with reactions or change experimental results. Distilled water is not necessarily perfectly pure.

Substances in natural water

Water from rivers, lakes and other natural sources is usually not chemically pure. It can contain substances dissolved in the water, solid particles suspended in it, and living organisms. These additional substances have different effects: some are beneficial, while others can be harmful.

Beneficial substances in natural water

Dissolved oxygen is needed by aquatic organisms for respiration. Some dissolved metal compounds supply mineral ions that organisms need for life. Other metal compounds, however, are toxic, so the effect depends on the substance present.

Potentially harmful substances in natural water

Plastics can damage aquatic life. Sewage can contain harmful microbes that cause disease. Natural water can also receive nitrates from fertilisers and phosphates from fertilisers and detergents.

Excess nitrates and phosphates can lead to deoxygenation: a decrease in the amount of dissolved oxygen in the water. They can encourage excessive growth of algae. When the algae die, microbes decompose them and use up dissolved oxygen. Less oxygen is then available for aquatic organisms to respire.

An impurity is simply something present in the water in addition to water itself. Its presence alone does not establish whether it is useful or harmful; that depends on what the substance is and its effect.

Domestic water treatment

Water is treated before it reaches homes to remove unwanted material and reduce the risk of disease. The treatment stages have different purposes: some remove solid particles, while others improve taste and smell or destroy harmful microorganisms.

Stages of domestic water treatment

1. Sedimentation: Water is left so that heavier solid particles settle at the bottom. The water can then be separated from this settled material.
2. Filtration: Water passes through a filter, which traps remaining suspended solids. These are particles that have not dissolved in the water.
3. Carbon treatment: Water passes over carbon, which removes dissolved substances that cause unpleasant tastes and odours. These substances stick to the surface of the carbon in a process called adsorption.
4. Chlorination: Chlorine is added to kill harmful microbes. This disinfects the water and reduces the risk of waterborne disease.

Filtration removes suspended particles, but it does not remove all dissolved substances. Carbon treatment and chlorination therefore have different jobs from the solid-removal stages: carbon improves taste and smell, while chlorine makes the water safer by destroying microorganisms.